The present invention relates to a control system for an electromagnetic clutch for an automatic transmission of a motor vehicle.
An automobile provided with a continuously variable belt-drive transmission with an electromagnetic clutch is disclosed in EP-A No. 151038. The electromagnetic clutch of the transmission is controlled by a control system to provide various operational modes such as a starting mode of a vehicle, reverse excitation mode, drag mode, and mode of lock-up engagement. One of the modes is selected in accordance with a position of a selector lever and driving conditions to control the electromagnetic clutch.
In the reverse excitation mode, a small reverse current flows in a coil of the clutch to completely release a driven member of the clutch from a drive member. In the start mode, the clutch current is increased in proportion to engine speed to start the vehicle. In the drag mode, a small clutch current flows to produce a drag torque exerted on the driven member, thereby reducing clearances formed in the power transmitting system in disengagement state of the clutch. In the clutch lock-up engagement mode, a lock-up current in response to the depression of an accelerator pedal is provided for entirely engaging the clutch.
Particularly, when the vehicle speed exceeds a predetermined speed, the start mode is changed to the clutch lock-up engagement mode, and the clutch lock-up engagement mode is changed to the drag mode during release of the accelerator pedal, after zero current (FIG. 3).
In the control system, the vehicle speed signal is obtained by a sensor operatively connected to an output shaft of the transmission or to a speedometer. Therefore, if the vehicle speed signal is not produced because of the failure of the sensor, cutting of a cable for the speedometer, disconnection of a wiring harness, or short circuit to the ground, the control system does not properly control the clutch current. It will be understood that the fact that the vehicle speed is zero means the vehicle is at a stop.
For example, when the accelerator pedal is depressed, the drag mode changes to the start mode. However, the start mode is not changed to the clutch lock-up engagement mode. Although the vehicle is normally started, the clutch torque changes in proportion to the engine speed because of the start mode. In such a state, it may cause the clutch to slip during the driving in accordance with driving conditions after the start of vehicle. Further, it will cause the deterioration of the clutch by excessive high temperature due to the slipping.
When the accelerator pedal is released, clutch mode is changed to the drag mode. Accordingly, the engine braking is not effected because of disengagement of the clutch, which results in lack of safety.
Further in accordance with the depression and release of the accelerator pedal at higher vehicle speed, the clutch is engaged or disengaged, which aggravates the driving characteristics.